Drain and rinse the garbanzo beans and black beans in a colander under cold running water; shake off excess water or pat dry with paper towels.
In a large bowl, combine the drained garbanzo beans, drained black beans, 1 cup chopped grape tomatoes, 1 large garlic clove (finely minced), ½ cup chopped red onion, ¼ cup chopped fresh parsley, and ¼ cup chopped fresh mint.
In a small bowl, whisk together 1 1/2 tablespoons olive oil and the juice from 1 medium lemon until the mixture is smooth and emulsified.
Pour the dressing over the beans and vegetables, add ½ teaspoon kosher salt and freshly ground black pepper to taste, and gently toss with a large spoon until everything is evenly coated.
Let the salad sit at room temperature for 30 minutes to allow the flavors to combine, then serve.